The present invention relates to a method for the disposal of waste water containing iron-cyanide complexes including ferricyanides by reducing in the presence of zinc ions to precipitate zinc ferrocyanide.
Iron-cyanide complexes, e.g. ferricyanides and ferrocyanides, are sometimes contained in waste water or sewage discharged from, for example, photographic processing laboratories, metal-plating plants, factories for cementation or nitriding of steels, warehouses for fumigation of vegetables and fruits, smelteries by the cyanide process and others. It is strictly required by laws and regulations that the concentration of cyanides in any waste water discharged to public waterways must be reduced to 1 ppm or below.
Several methods are known in the prior art for the removal of iron-cyanide complexes from waste water including the methods of chemical, electrochemical or photochemical decomposition, physical or physicochemical methods of concentration by ion-exchange, electrosmosis, reverse osmosis and the like, adsorption methods by use of an adsorbent such as an active charcoal and precipitation methods by use of a precipitant.
Each of these prior art methods is, however, not quite satisfactory in one or more respects. For example, the method of decomposition must be performed under very critical reaction conditions due to the high chemical stability of the iron-cyanide complexes and no satisfactorily high velocity of decomposition can be obtained even by the consumption of large quantities of expensive energies such as electric and light energies and use of a special apparatus so that the method is economically disadvantageous. The methods of physical concentration are defective due to the outstanding expensiveness of the apparatuses used in the method in addition to the problem that the thus concentrated waste water relative to the concentration of the iron-cyanide complexes must be subjected to a secondary treatment by the method of decomposition, precipitation and the like to remove the complexes before the waste water is discharged. Further, the method of adsorption is practically not feasible because of the unavailability of an inexpensive adsorbent having a large capacity and good selectivity for the adsorption of cyanide complexes.
The method of precipitation is suitable for the treatment of large volumes of waste water. There are several problems, however, even in the prussian blue method, which is the only method now practically undertaken, by use of an iron salt as the precipitant. For example, the process is not free from the influences of the atmospheric oxygen and the effect of variation in the value of pH affecting the solubility behavior of the precipitates so that a very strict process control is indispensable to obtain reliable results even by setting aside the inherent problem that the method is not sufficiently effective to satisfy the requirement of the regulations for the maximum cyanide concentration in waste water which sometimes exceeds a few ppm even in the most favorable cases by the precipitation method.
A modification of the precipitation method is proposed in Japanese Patent Publication No. 57-5598 in which the iron-cyanide complexes including ferricyanide compounds in a waste water is reduced by a reducing agent in the presence of zinc ions so as to convert the iron-cyanide complexes into zinc ferrocyanide which is precipitated and removed by filtration.
The reducing agents used in the above proposed method include thiosulfates, sulfites, pyrosulfites, dithionites, salts of hydrazine, salts of hydroxylamine and the like. This method is advantageous in comparison with the method in which the ferricyanide ions are first reduced by the reducing agent into ferrocyanide ions followed by the precipitation of zinc ferrocyanide by use of a zinc salt in respect of the rapidness of the treatment and the completeness of decrease in the residual cyanide concentration which can be 1 ppm or smaller without particular difficulties. This known method is, however, not quite satisfactory because the above named reducing agents are each not free from one or more problems. For example, the salts of hydrazine and hydroxylamine are too expensive to ensure wide applicability of the method in practice. The sulfites, pyrosulfites and dithionites are highly susceptible to the oxidizing influence of atmospheric oxygen dissolved in the waste water. The thiosulfates are, on the other hand, relatively free from the oxidizing influence of the dissolved oxygen but this advantage is cancelled by the disadvantage that the velocity of the precipitation of zinc ferrocyanide is relatively low with a reducing agent of this class.
As is described above, the above mentioned reducing agents have their respective advantages and disadvantages so that each of them requires particular treatment conditions in the practical application of the method. When sodium sulfite is used as the reducing agent, for example, the reduction of the ferricyanide ions is quite incomplete to leave a large amount of the unreduced ferricyanide ions as is shown by the comparative example given later, by the addition of an equimolar amount of the reducing agent such as sodium sulfite, sodium hydrogensulfite and sodium pyrosulfite and 2.5 times by moles of a zinc salt to the ferricyanide ions in the waste water so that the amount of the reducing agent must be increased greatly, for example, up to 8.times.10.sup.-4 moles/liter or larger for the ferricyanide ion concentration as in the comparative example. When sodium thiosulfate is used as the reducing agent in an equimolar amount to the ferricyanide ions in the waste water, a treatment time of as long as 120 minutes or longer is required to decrease the residual cyanide concentration to 1 ppm CN or smaller as is shown in the comparative example given later so that practical applicability can hardly be found industrially with this method.
As is mentioned above, furthermore, sulfites are highly susceptible to the oxidation by the atmospheric and dissolved oxygen to the loss of their reducing activity so that certain measure should be undertaken to prevent oxidation of the sulfite ions by the oxygen. It is known in the art that alcohols, or in particular, benzyl alcohol are very effective as an inhibit or of the air oxidation of sulfite ions. Unfortunately, the effectiveness of benzyl alcohol as an inhibitor of air oxidation of sulfite ions can hardly be exhibited when it is used in combination with a sulfite in the presence of a zinc salt for the treatment of ferricyanide ions.
Thus, it has been eagerly desired to develop an effective method for the disposal of waste water containing iron-cyanide complexes including ferricyanides in a process of reduction of the ferricyanide ions to ferrocyanide ions followed by precipitation of zinc ferrocyanide in the presence of a zinc salt by use of a relatively small amount of the reducing agent within a relatively short treatment time.